1. Field of the Invention
The present invention relates to a write head having a first pole piece with a frequency dependent variable effective throat height and, more particularly, to a high data rate write head which writes well (hard) within a track without overwriting adjacent tracks.
2. Description of the Related Art
The heart of a computer is a magnetic disk drive which includes a rotating magnetic disk, a slider that has read and write heads, a suspension arm above the rotating disk and an actuator arm that swings the suspension arm to place the read and write heads over selected circular tracks on the rotating disk. The suspension arm urges the slider into contact with the surface of the disk when the disk is not rotating but, when the disk rotates, air is swirled by the rotating disk adjacent an air bearing surface (ABS) of the slider causing the slider to ride on an air bearing a slight distance from the surface of the rotating disk. When the slider rides on the air bearing the write and read heads are employed for writing magnetic impressions to and reading magnetic field signals from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
A write head typically employs ferromagnetic first and second pole pieces which are capable of carrying flux signals for the purpose of writing the magnetic impressions into the track. Each of the first and second pole pieces has a pole tip, a yoke and a back gap with the yoke being located between the pole tip and the back gap. The pole tips are located at the ABS and the back gaps are magnetically connected at a recessed location within the write head. At least one coil layer is embedded in an insulation stack between the yokes of the first and second pole pieces. A nonmagnetic write gap layer is located between the pole tips. Processing circuitry digitally energizes the write coil which induces flux signals into the first and second pole pieces. The flux signals bridge across the write gap layer at the ABS so as to write the aforementioned magnetic impressions or bits into the track of the rotating disk. The thinner the thickness of the write gap layer, the greater the number of bits the write head can write into the track.
The first and second pole pieces are typically fabricated by frame plating. Photoresist is employed to provide the frame and a seed layer is employed to provide a return path for the plating operation. A typical sequence for fabricating a pole piece is to sputter clean the wafer, sputter deposit a seed layer, such as nickel iron, on the wafer, spin a layer of photoresist on the wafer, light-image the photoresist layer through a mask to expose areas of the photoresist that are to be removed (assuming that the photoresist is a positive photoresist), develop the photoresist to remove the light-exposed areas to provide an opening in the photoresist and then plate the pole piece in the opening up to a desired height.
The magnetic moment of each pole piece is parallel to the ABS and to the major planes of the layers of the write head. When the write current is applied to the coil of the write head the magnetic moment rotates toward or away from the ABS, depending upon whether the write signal is positive or negative. When the magnetic moment is rotated from the parallel position, the aforementioned magnetic flux fringes across the write gap layer between the first and second pole pieces impressing a positive or negative bit in the track of the rotating magnetic disk. As the write current frequency is increased the linear bit density is also increased. An increase in the linear bit density is desirable in order to increase the aforementioned areal density which provides a computer with increased storage capacity.
A write head is typically rated by its areal density which is a product of its linear bit density and its track width density. The linear bit density, which is dependent on the thickness of the write gap layer and the data rate of the write head, is the number of bits which can be written per linear inch along the track of the rotating magnetic disk and the track width density, which is dependent on the track width of the write head, is the number of tracks that can be written per inch along a radius of the rotating magnetic disk. The linear bit density is quantified as bits per inch (BPI) and the track width density is quantified as tracks per inch (TPI). Efforts over the years to increase the areal density of write heads have resulted in computer storage capacities which have increased from kilobytes to megabytes to gigabytes.
Efforts still continue to obtain higher BPI and TPI in order to improve the areal density of a write head. The throat height of a write head plays a key role in obtaining a desirable BPI and a desirable TPI. The throat height of a write head is the length of a pole tip portion of a first or second pole piece of the write head from the ABS to a recessed location within the head where the first and second pole pieces commence to separate after the ABS. The recessed location is referred to in the art as the zero throat height (ZTH). The greater the throat height the better the TPI and the less the throat height the better the BPI. The reason for this is because the write signals of the write head vary in frequency. For instance, if a write head is writing a series of ones into the track of a rotating magnetic disk the write signals are at the highest frequency, if a one is followed by one or more zeroes the frequency is lower and if the write signal is DC, for the purpose of erasing a track, the write signal is at the lowest frequency. When the write frequency is high the permeability of the magnetic material of the pole pieces is low and has high reluctance and when the write frequency is low the magnetic material has high permeability and low reluctance.
Assuming that the write head is designed only for the purpose of effectively writing high frequency write signals, such as one gigabit (GB) per second, into the track of the rotating magnetic disk it is desirable that the throat height be short, such as 0.5 xcexcm from the ABS to the ZTH. With this arrangement the write head can have a high data rate since the write signals are strong and can be easily read by the read head of the magnetic head assembly. Assuming that the write signal frequency of such a head is low, the write head will still effectively write well along the length of the track but will cause a problem on each side of the track. The write head will cause a large erase band and/or a high level of adjacent track interference (ATI) on each side of the track that is being written. This is caused by a large corner field which emanates from bottom corners of the second pole tip at the ABS. This corner field spreads outwardly from the track width (TW) and magnetically affects the areas on each side of the track being written. The thickness of the second pole tip at the ABS also contributes to adjacent track interference by overhanging adjacent tracks, especially at the outer track locations of the rotating magnetic disk. In essence, when the write head is designed for high data rates only, the write head becomes overly efficient at low frequency data rates causing too much flux to fringe between the first and second pole tips because not enough flux is being shunted between the first and second pole pieces.
Now assuming that the write head is designed to provide a small erase band and minimal adjacent track interference at low frequency, the throat height would be increased, such as to 1.5 xcexcm from the ABS to the ZTH. Accordingly, when the write frequency is low there is more of the throat height to cause a shunting of the flux between the first and second pole pieces to prevent unacceptable erase bands and adjacent track interference on each side of the track being written. Unfortunately, the larger throat height of such a write head will not write as effectively, especially at high data rates, as the write head with the shorter throat height. This is because more flux is being shunted between the first and second pole pieces because of the longer throat height and less flux is bridging between the first and second pole tips at the ABS to write strongly or hard within the track being written.
The two assumptions described hereinabove demonstrate the dilemma in designing a throat height for a high data rate write head to obtain strong signals along the track being written while avoiding unacceptable erase bands and adjacent track interference on each side of the track being written.
The present invention overcomes the aforementioned problems by providing a uniquely shaped throat so that the effective throat height is a function of the frequency. The effective throat height is smaller at high frequency and the effective throat height is larger at low frequency. In the invention the pole tip portion of the first pole piece is configured with first and second components wherein the first component forms a portion of the ABS and the second component is recessed from the ABS and is magnetically connected to the first component. The second component has a width that is less than a width of the first component wherein the widths are parallel to the ABS and parallel to major thin film planes of the layers of the sensor. Accordingly, the second component is constricted with respect to the first component and has a higher efficiency roll-off than the first component, which means the flux carrying efficiency of the second component decreases as the write signal frequency increases. It can be assumed that the first component has a length into the head of 0.5 xcexcm and the second component, which is an extension of the first component into the head, has a length of 1.0 xcexcm which gives a total throat height of 1.5 xcexcm. Assuming that the write signal frequency is high, the permeability of the second component degrades and is less capable of carrying flux to the write gap or shunting flux to the second pole piece layer. Accordingly, the effective throat height is 0.5 xcexcm and since the first component is designed to carry the desired high write signal frequency the head will write well into the track being written without unacceptable erase bands and adjacent track interference on each side of the track being written. When the write signal frequency becomes low the permeability of the second component is high and the effective throat height is 1.5 xcexcm. In this mode, more flux is shunted between the second component and the second pole piece so as to prevent an over-amount of flux at the write gap to prevent the aforementioned unacceptable erase band and adjacent track interference. Accordingly, with the present invention the strength of the field signals being written into the track of the rotating magnetic disk is substantially constant throughout the operating frequency range, without the problem of unacceptable erase bands and adjacent track interference on each side of the track being written.
An object of the present invention is to provide a throat for a first pole piece of a write head which has a frequency dependent variable throat height.
Another object is to provide a write head wherein the strength of the field signal being written into a circular track of a rotating magnetic disk is substantially constant over a large frequency range without unacceptable erase bands and adjacent track interference on each side of the track being written.
Other objects and attendant advantages of the invention will be appreciated upon reading the following description taken together with the accompanying drawings.